1. Field of the Invention
The present invention relates to an imaging device for acquiring an image of an object located at a predetermined position by utilizing light within a particular wavelength range and, in particular, to an imaging device of close contact type for acquiring an image of an object in a state that the object is almost in close contact to an image sensor.
2. Description of Related Art
Imaging devices are widely used that acquire images of manuscripts such as printed images, documents, and bar code images. As examples of such imaging devices, imaging devices of close contact type are known in which image acquisition is performed in a state that a line image sensor having the same width as a manuscript to be read is located almost in close contact to the manuscript. Such imaging devices of close contact type have a relatively short distance from the manuscript to the image sensor, and hence have a compact configuration in comparison with other types employing a reduction optical system.
When an image sensor is merely simply arranged almost in close contact to a manuscript without employing a lens or the like for forming an image of the manuscript, light beams from a particular point on the manuscript do not exclusively reach one pixel of the image sensor. That is, the light beams are incident onto a plurality of pixels in its periphery in addition to the intrinsically intended pixel. Thus, a clear image is not obtained. Thus, even in an imaging device of close contact type, a cylindrical rod lens having a refractive index distribution that decreases from the center toward the periphery is arranged between the image sensor and the manuscript so that incidence of light onto the image sensor is limited such that light from a particular point on the manuscript should reach one imaging area of the image sensor almost in one-to-one correspondence.
Further, as examples of imaging devices in which incidence of light onto the image sensor is limited, devices are known that employ a filter for limiting the incident angle of light onto the image sensor in order that the image sensor should be used in a state that the light acceptance area of the image sensor is divided into a plurality. Known examples include: an image sensor in which a shielding block is arranged in accordance with light acceptance areas to be formed (JP-A-2007-121631); and an imaging device in which a grid plate having slits is provided so that the light acceptance area is divided into rectangles (JP-A-2007-299085). Another known example is a film camera in which a multi-hole plate having a large number of through holes is provided so that the incident angle of light onto the light acceptance area is limited such that parallel light beams alone that are perpendicularly incident onto the light acceptance area reach the film (JP-A-2004-151124).
In an image sensor, photodiodes perform photoelectric conversion in individual pixels so as to obtain an image of an object. Here, the photo-diodes are located at a level deeper than the surface of the image sensor. Thus, even in an imaging device of close contact type, the object such as a manuscript cannot exactly be in close contact to the photodiodes, and hence a gap is unavoidably formed between the object and the photodiodes. When a gap is formed between the object and the photodiode, light from a particular point on the object not only enters the intrinsically intended photodiode but also enters obliquely a plurality of adjacent photodiodes. This causes noise and false signals. Thus, for the purpose of fine image acquisition, the incident angle of light onto the image sensor need be limited such that light from a particular point on the object enters one photodiode alone.
In order to avoid a situation that light from a particular point on the object enters not only the intrinsically introduced photodiode (or light acceptance area composed of photodiodes) but also adjacent photodiodes, the incident angle of light onto the image sensor need be limited, for example, by using a rod lens widely employed in an imaging device of close contact type or alternatively by using a filter for dividing the imaging area with through holes as described in JP-A-2007-121631 and JP-A-2007-299085.
Nevertheless, when a rod lens is to be employed, a relatively large assembling space is necessary in the thickness direction and hence causes difficulty in further thickness reduction. Further, in order that the rod lens should form an erect unity-magnification image of the object, the length of the rod lens, the intervals between the object, the rod lens, and the image sensor, and the like need be controlled strictly. This causes difficulty in alignment.
Further, a filter having through holes such as slits is fabricated from a metal plate or the like. Nevertheless, with decreasing through hole diameter and with decreasing through hole interval, difficulty increases in the processing of fabrication through holes. Thus, even when such minute through holes can be fabricated by machining, a high cost is caused in the filter fabrication. Further, such a filter having minute through holes has a difficulty that arrangement need be performed in such a manner that the positions of the through holes are precisely aligned with the image sensor.
When a photolithography technique is employed, a filter having minute through holes can be fabricated. Nevertheless, the same difficulty remains that arrangement need be performed in such a manner that the positions of the through holes are precisely aligned with the image sensor.
In filters having small-diameter through holes as described above, clogging is easily caused by dust, particulates, and the like in the fabrication process or at the time of assembling into a close contact type imaging device. Then, dust having once entered the through holes is difficult to be removed. Thus, in the filter having small-diameter through holes, sufficient care need be taken concerning the peripheral environment and the like at the time of handling. Further, the production yield is unsatisfactory in filters having through holes and close contact type imaging devices employing this filter. Thus, stable mass production is difficult.
Furthermore, when a part of incident light is reflected by (or transmitted through) the inner wall of each through hole, a light beam is generated that reaches outside a region defined by the diameter of the through hole. Thus, antireflection processing or the like is necessary in the inner wall of the through hole. Nevertheless, in the case of a minute through hole whose fabrication itself is difficult, a special processing onto the inner wall is remarkably difficult to be performed.
On the other hand, in place of assembling a filter having minute through holes, an arrangement of through holes is directly built in the image sensor itself by a so-called MEMS method, the necessity of processing of aligning the through holes with the image sensor is avoided. Nevertheless, when an array of through holes is to be built in the image sensor itself, the structure and the manufacturing process itself of the image sensor are affected and hence a remarkable cost increase is caused. Further, another problem arises that the process of fabricating through holes having a sufficient height in the close contact type imaging device requires a remarkable etching time and hence the image sensor having a built-in arrangement of through holes is not suitable for mass production.